The present invention relates to anhydrous crystalline forms and solvates of the antiviral compound 5,6-dichloro-2-(isopropylamino)-1-(β-L-ribofuranosyl)-1H-benzimidazole (also known as 1263W94; a compound of formula (I)), pharmaceutical formulations comprising anhydrous crystalline forms and solvates of this compound, and their use in therapy.
5,6-Dichloro-2-(isopropylamino)-1-(β-L-ribofuranosyl)-1H-benzimidazole (1263W94) is a benzimidazole derivative useful in medical therapy. WO96/01833 discloses 5,6-dichloro-2-(isopropylamino)-1-(β-L-ribofuranosyl)-1H-benzimidazole and its use for the treatment or prophylaxis of viral infections such as those caused by herpes viruses. The compound as disclosed in WO96/01833 is an amorphous, non-crystalline, hygroscopic material.
The structure of 5,6-dichloro-2-(isopropylamino)-1-(β-L-ribofuranosyl)-1H-benzimidazole, a compound of formula (I) is shown below:

We have now found that the compound of formula (I) may be prepared in novel crystalline forms and solvates. The novel crystalline forms and solvates of the present invention are more thermodynamically stable than the amorphous form disclosed in WO96/01833. The novel crystalline forms and solvates have the further advantage that they are essentially non-hygroscopic. These crystalline forms and solvates have good storage properties and can be readily formulated into pharmaceutical compositions such as tablets, capsules, and liquid systems. The crystalline forms and solvates may be characterized by their X-ray powder diffraction patterns.